A folic acid-antagonistic folic acid analogue means a substance which has a chemical structure similar to folic acid, but inhibits partially or totally the actions of folic acid or its derivatives. A variety of such substances have been synthesized and some of them have been applied to the chemotherapy for cancers such as leukemia or malignant tumors.
It has been known that a cytotoxic conjugate of a folic acid-antagonistic folic acid analogue with an antibody is formed by linking methotrexate with the antibody using a water-soluble carbodiimide as a condensation agent or using an active ester of methotrexate (see P. N. Kulkarni et al.: Cancer Research 41, 2700-2706, 1981). Further, M. C. Garnet disclosed a method of conjugating methotrexate with an antibody via serum albumin as an intermediary (Int. J. Cancer, 31, 661-670, 1983). Neither, however, the process for conjugation of a folic acid-antagonistic folic acid analogue with an antibody or its fragment via an oligopeptide nor such a cytotoxic antibody conjugate have been known. Conventional conjugates resulting from direct conjugation of methotrexate with an antibody is unsatisfactory in potency and selectivity of cytotoxicity and antitumor activity, while oligopeptide-mediated conjugates seem to meet the expectation of resolving these problems.